Within the offshore oil and gas recovery industry the possibility to store fluids such as crude oil, chemicals, produced water on the sea bottom provides a huge increase in available storing space compared to storing on platforms and a more stable storing environment compared to storing on vessels such as FPSO's (Floating production storage and off loading). The subsea storage with a flexible storage bag within a protection structure surrounding the bag is a promising solution to make use of this possibility in a safe way. Subsea storing also plays a part in the possibility of providing an oil and/or gas recovery plant fully installed subsea with no offshore top side installations except from a buoy.
Also in other storing systems, not necessarily arranged subsea, it may in some situations be advantages to employ a flexible bag arranged within an outer protection structure. Such situations include storing in vessels or platforms where a first fluid is stored in the flexible bag and the flexible bag is surrounded by a second fluid.
When storing none pure fluids the storing of an initial homogeneous fluid may over time result in separation of the different components, for instance due to density differences, also changes in temperature may result in formation of an inhomogeneous fluid. In some situations one may also wish to store an inhomogeneous fluid. In all these situations there is a desire to be able to fully empty the storage without risking a build up of one component in a rest volume of the storage after several circles of filling and emptying.